1. Field of the Invention
The present invention relates to paraboloidal reflector antennas, and more particularly to dipole feeds for such antennas.
2. Description of the Prior Art
Paraboloid antennas, consisting of a dish-shaped surface illuminated by a feed horn mounted at the focus of the reflector, are commonly used in microwave communication applications involving line-of-sight transmission facilities operating at frequencies higher than 960 MHz. Since the performance of this type of antenna is closely related to its feed, the feed has to be designed for high antenna efficiency and low cross-polarization, which can be achieved with a feed having symmetric E-plane and H-plane radiation patterns.
Dipole feeds have been used extensively as the feeds for paraboloidal reflector antennas, particularly where such antennas have radar and low frequency applications. The dipole, being approximately one-half wavelength long, is split at its electrical center for connection to the transmission line. The radiation pattern of the dipole is a maximum at right angles to the axis of the antenna. In virtually all current designs, the dipole feed is used with a reflecting disk or a reflecting rod which propagates the radiation field towards the reflector. Such designs are structurally simple and thus relatively rugged and easy to fabricate, but have the disadvantage of generating unequal E-plane and H-plane patterns, which illuminate the reflector surface in an asymmetric manner and thereby cause high reflector cross-polarization, high side and back lobe levels, and a low reflector gain factor.
More recently, a common design for the feed makes use of a circular waveguide having a corrugated flange to improve the efficiency thereof. The geometry of such a feed is, however, relatively complex, and consequently the feed is expensive and difficult to fabricate. In addition, the corrugated feed must be supported by struts that cause aperture blockage, which normally reduces the antenna gain and increases the cross-polarization and the side lobe levels.
Accordingly, it is desirable to be able to design a low cost dipole feed which would offer weight and cost advantages over existing designs, especially at low microwave frequencies. One such improvement to the design of dipole feeds was recently described by Kildal in "Dipole-Disk Antenna with Beam-Forming Ring", IEEE Transactions on Antennas and Propagation, July 1982, Vol AP-30, p. 529, whereby an additional ring in front of the dipole is used to improve the radiation pattern. This dipole feed, however, provides relatively narrow beams and also emits a comparatively high level of back radiation.